Hi Allan; I with you, I normally downsize the interconnects in parallel strings to 2/0, but keep them the same as the main cables on anything that carries full current, as Phil made clear. On golf cart batteries for instance, two 4/0 parallel lugs won't fit under one battery post, now that most manus shortened their posts. Also 4/0 cables can really strain those posts; I saw a couple of systems with 4/0 THHN used (NEC compliant) but the lead post ripped off the battery after several years from the bend put on the cable (creating a much more dangerous and unfused problem BTW) Some of the sealed battery manus (B&B for instance) give max current output vs time charts, and that not only gives you something for cable sizing, but also is an eye opener, when expecting a single string of two 12 volt batteries to run a 4 Kw inverter. A battery that can only produce 250 amps for less than 5 sec, doesn't deserve a cable sized for 24 hr continuous at that output.
R. Walters [email protected] Solar Engineer On Feb 19, 2010, at 10:03 PM, Allan Sindelar wrote: > Phil, > Given the scenario that Mick presented, your advice is sound, and I'd do as > you suggest. But by changing the scenario just a bit, it brings up a separate > but related issue. > > Mick's scenario describes two 12V batteries in series, or a single string. In > that case all current flows through the one string, so a series interconnect > would need to be sized equal to the battery cables, in this case 4/0. But if > there were two (or more) parallel strings, would the same size requirement > apply? That is, if a battery bank had two strings, in theory each would carry > 180 max amps (your example, 8,000/22, shared by 2 strings), which is within > the ampacity of 2/0 interconnects. In reality, this only applies if current > is equally spread among strings. Is it? > > It seems to me that a properly wired and torqued set will split current paths > equally until a cell fails, either prematurely or when the set approaches end > of life. If there are three strings, then failure of one cell/string in this > example would still not exceed ampacity on the remaining 2/0 strings. > > As with many issues, the goal is to find a balance between safety and > performance versus cost and worst-case accommodation. I will always use 4/0 > with one cell string and a 250A breaker, but I have never seen a real-world > problem using 2/0 interconnects, both series and parallel, with 2 or more > strings. Phil, what do you say? Others? > > Allan Sindelar > [email protected] > NABCEP Certified Photovoltaic Installer > EE98J Journeyman Electrician > Positive Energy, Inc. > 3201 Calle Marie > Santa Fe, New Mexico 87507 > 505 424-1112 > www.positiveenergysolar.com > > > Phil Undercuffler wrote: >> >> The interconnects are part of the circuit. Typical flexible cables used >> with batteries are THW rated (75C column), so in free air 2/0 interconnects >> would be good for 265 amps, before any temperature derates. Therefore, >> you'd be safe from a NEC point of view. However, if the inverter >> manufacturer has recommended 4/0 cables, then that recommendation would >> apply to all cables in the circuit. Battery based inverters are asked to >> surge many times their rated capacity -- a 24V 4kW inverter surging to 2x >> capacity (pretty normal circumstances) is going to draw over 360 amps (8,000 >> / 22), even if only for a short period of time. You don't want your >> interconnects to be the weak link in the system, causing shutdowns due to >> undervoltage unnecessarily. >> >> I'd use 4/0. >> >> Phil Undercuffler >> Conergy >> >> >> >> >> >> On Fri, Feb 19, 2010 at 12:01 PM, Mick Abraham <[email protected]> wrote: >> Example: 12 two volt battery cells in one 24 volt string. 4kW >> inverter/charger has 250 amp DC breaker size and 4/0 cables...as the >> inverter folks would want. >> >> So...do the "cell to cell" interconnect cables have to also be 4/0? In the >> example above, 2/0 interconnects would suffice from the standpoint of basic >> safety...and even from the voltage drop standpoint...right? >> >> Inverter company people may be best qualified to answer this, but any >> replies will be appreciated. While I'm at it: do the inverter folks request >> oversize cables because they want more capacitance on the DC input...or what? >> >> Jolliness, >> >> Mick Abraham, Proprietor >> www.abrahamsolar.com >> >> Voice: 970-731-4675 >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: [email protected] >> >> Options & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> List-Archive: >> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> www.re-wrenches.org/etiquette.htm >> >> Check out participant bios: >> www.members.re-wrenches.org >> >> >> >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: [email protected] >> >> Options & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> List-Archive: >> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> www.re-wrenches.org/etiquette.htm >> >> Check out participant bios: >> www.members.re-wrenches.org >> >> > _______________________________________________ > List sponsored by Home Power magazine > > List Address: [email protected] > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org >
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